Devices for scrubbing polluted gas are known which comprise trickle walls that are inclined toward each other and define an opening therebetween; means for causing the scrubbing liquid to trickle or stream down these walls; and means for providing circulation of the polluted gas to be scrubbed downwardly from above through the opening between the trickle walls. With such an arrangement, the scrubbing liquid trickles down the inclined walls, flows through the opening between the walls, and forms a liquid sheet or curtain through which the polluted gas must pass before being discharged to a location outside of the paint room.
In prior art installations it has been proposed to provide that the opening between the trickle walls is disposed above a transverse wall with a continuous contour or extent equal to, or preferably larger than, that of the opening between the trickle walls. Thus, with this arrangement, the scrubbing liquid strikes this transverse wall and is diverted to the side thereby before falling into a collection basin or receptacle below. By virtue of the provision of this transverse wall, it is possible to obtain a sheet of scrubbing liquid of predetermined dimensions. However, when purified gas is extracted from the side, this arrangement has the drawback of causing the droplets of scrubbing liquid to be carried off with the purified gas, because the scrubbing liquid is turned into a spray when striking the transverse wall.
In order to avoid this drawback, it has been proposed in the prior art to surround the opening between the trickle walls, and the transverse wall, with walls defining a trickle passage having vertical axis, the trickle passage being open at the bottom end thereof and the walls being affixed at the tops thereof to the undersides of the trickle walls.
New regulations have been promulgated with respect to atmospheric pollution which are stricter than former regulations, and as a consequence, the scrubbing devices, or scrubbers, for the air drawn from paint rooms must now operate with greatly reduced charges of paint in the circulating air so as to diminish the discharge into the atmosphere of paint particles whcih are not collected by the scrubber. Accordingly, paint particles of very small dimensions (smaller than 1 micrometer) are used and these are the most difficult to collect. Collection of these paint particles requires the consumption of more energy as the size of the particles decreases resulting in a scrubber with a greater loss of charge into the circulating air.
To satisfy the requirements mentioned above, scrubbers used in paint rooms have operated with losses of between 80 and 120 mm (water column). Under these conditions, the scrubbers are not very noisey (approximately 82 dBA), the carrying off of droplets of scrubbing liquid is limited to acceptable ratios, and the paint particle content in the air discharged into the atmosphere is between 5 and 10 mg/m.sup.3 depending on the types of paints being used, the conditions of application of the paint (manual gun, robot gun, pneumatic, or electrostatic) and the quantities of paint being sprayed.
However, when prior art scrubbing devices such as those described above have been operated with higher losses of charge, e.g., 200 to 300 mm (water column) in order to satisfy the requirements of the new air pollution regulations, in other words, so that the paint particle content in the air drawn from the paint rooms is equal to or lower than 3 mg/m.sup.3, for example, a number of problems have developed.
In particular, it has been shown that certain scrubbers do not operate correctly when a certain loss of charge, referred to as the critical loss of charge, is exceeded, and the emission of paint particles, instead of continuing to decrease, suddenly increases to high levels (10 to 20 mg/m.sup.3).
It has also been shown that while certain other scrubbers operate correctly, with the effectiveness of collection provided thereby increasing in relation with the loss of charge, other problems have occurred which are incompatible with industrial usage. These problems include an intense operational noise, of 90 to 95 dBA, which is above the permitted standard, and a large amount of droplets of scrubbing liquid being carried away, these droplets carrying with them paint particles collected by the scrubbing liquid. The result is a deposit of paint on the walls of the casings and fans for extraction of the air from the paint room, and this requires frequent cleaning, necessitating stopping the installation, and discharging of droplets of scrubbing liquid charged with paint particles into the atmosphere.
Thus, despite the creation of heavy losses of charge in the prior art scrubbers, requiring great energy consumption, a satisfactory result has not been obtained which overcomes all of the various problems discussed above.
As a solution, the attachment of a second scrubber to the existing scrubber has been proposed in order to avoid these problems and drawbacks. Such a scrubber can be placed either beneath the first scrubber, or outside the paint spray room, in the chimney for evacuation of air from the room. However, such an additional scrubber is cumbersome and its use requires an additional investment for the construction of the paint room.